JWST - European Space Agency

JWST

Inspired by the success of the Hubble Space

Telescope, NASA, ESA and the CanadianSpace Agency have collaborated since 1996

on the design and construction of a scientificallyworthy successor. Due to be launched fromKourou in 2013 on an Ariane-5 rocket, theJames Webb Space Telescope is expected tohave as profound and far-reaching an impact onastrophysics as did its famous predecessor.

IntroductionAstronomers cannot conduct experi-ments on the Universe, instead theymust patiently observe the night sky asthey find it, teasing out its secrets onlyby collecting and analysing the lightreceived from celestial bodies. Since thetime of Galileo, the foremost tool ofastronomy has been the telescope,feeding first the human eye, and laterincreasingly sensitive and sophisticatedinstruments designed to record anddissect the captured light.

With the coming of the Space Age,astronomers soon began sending theirtelescopes and instrumentation intoorbit, to operate above the constrainingwindow of Earth’s atmosphere. One ofthe most successful astronomical

Peter Jakobsen & Peter JensenDirectorate of Scientific Programmes, ESTEC,Noordwijk, The Netherlands

esa bulletin 133 - february 2008 33

observatories launched into space todate has been the NASA/ESA HubbleSpace Telescope (HST), which operatesat visible wavelengths, with excursionsto the ultraviolet and near-infrared.

Although HST’s 2.4 m-diametercollecting mirror is dwarfed by many farlarger modern telescopes on the ground,from its vantage point above theblurring turbulence of the upperatmosphere HST has provided thedeepest and clearest views yet of nearlyall types of astronomical object. Someof the more spectacular HST imageshave even achieved iconic status with thegeneral public.

Launched in April 1990, HST hassince been repaired, maintained and itsinstruments upgraded during foursubsequent visits by the Space Shuttle.HST is presently scheduled to undergoits fifth and final servicing mission inAugust 2008.

Since 1996, NASA, ESA and theCanadian Space Agency (CSA) havecooperated on designing andconstructing a worthy successor to theHubble Space Telescope. Knowninitially as the Next Generation SpaceTelescope, the project was renamed in2002 as the James Webb Space Telescope(JWST) after the former NASAadministrator led the US agency duringone of the most impressive projects inhistory – landing a man on the Moon.

Although in several aspects JWSTrepresents a radical departure from itspredecessor, the astronomical capa-bilities of the JWST telescope and itsinstruments are very much driven by thescientific successes of HST, especiallyconcerning exploration of the earlyUniverse.

The ObservatoryThe JWST observatory consists of a6.55 m-diameter telescope, optimised fordiffraction-limited performance in thenear-infrared (1–5 μm) and mid-infrared(5–28 μm) wavelength regions. Thereason for the large telescope apertureand shift to the infrared is the desire tofollow the contents of the faintextragalactic Universe back in time, tothe epoch of ‘First Light’ and theignition of the very first stars.

Nonetheless, like its predecessor,JWST will be a general-purposeobservatory and carry a full suite ofastronomical instruments capable ofaddressing a broad range of outstandingproblems in current astrophysics. Incontrast to HST, however, JWST will beplaced into an orbit, known as an ‘L2’halo orbit, some 1.5 million km fromEarth and away from the Sun in deepspace. This means that it is not designedto be serviceable after launch.

JWST will carry a total of fourscientific instruments whose capabilities

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Who was James Webb?

James E. Webb (1906-92) was NASA’s

second administrator. Appointed by President

John F. Kennedy in 1961, Webb organised

the fledgling space agency and oversaw the

development of the Apollo programme until

his retirement a few months before Apollo 11

successfully landed on the Moon. Although

an educator and lawyer by training, with a long career in public

service and industry, Webb can rightfully be considered the

father of modern space science. During his tenure as

administrator, Webb insisted that NASA not only focused on

manned spaceflight, but also embarked on a balanced

programme of scientific research. As a result, by the time of his

retirement, NASA had already launched some 75 scientific

missions in astronomy, planetary exploration and space science.James E. Webb escorts President John F. Kennedy during a visit to a NASA centre in 1963

Telescope elements: (top to bottom) scaled-down testbed;prototype mirror backplane segment; and mirror segmentattachment and adjustment mechanism

together span the full contents of anastronomer’s toolbox:– NIRCam: a wide field (2.2 x 4.4

arcmin) near-infrared cameracovering the wavelengths 0.6-5 μm

– NIRSpec: a wide field (3.5 x 3.5

arcmin) multi-object near-infraredspectrometer covering the wave-lengths 0.6-5 μm at spectral resolu-tions of R~100, R~1000 and R~2700

– MIRI: a combined mid-infraredcamera (1.4 x 1.9 arcmin) andspectrograph (R~100 and R~2000)covering the wavelengths 5-27 μm

– FGS/TFI: a fine guidance camerathat also carries a near-infraredtunable filter imaging capability (2.3 x2.3 arcmin; R~100).

The telescope and its instruments areto be cooled in bulk down to –240ºC, atemperature determined to avoidtelescope self-emission in the near-infrared and the required operatingtemperature of the mercury-cadmium-

telluride (HgCdTe) detector arraysemployed by the three near-infraredinstruments. Cooling is achieved bykeeping the telescope and itsinstrumentation in perpetual shadowbehind a large deployable sunshade.Further cooling of the MIRI instrumentto below –263ºC is achieved with adedicated mechanical cooler.

The telescope is made up of 18hexagonal segments, and is specified toyield diffraction-limited performance atwavelengths above 2 μm in the near-infrared. In order to fit into the shroudof the ESA Ariane-5 launcher, the 6.55 mprimary mirror needs to be folded anddeployed with the secondary mirroronce in orbit. The precise positioning ofeach telescope segment is individually

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JWST

Artist impression of the deployed JWST in orbit showing the 6.55 m segmented telescope mirror and matching sunshade (Northrop-Grumman)

Comparison of Hubble and JWST mirrors (NASA)

controllable in six degrees of freedom,and the radius of curvature of eachsegment can also be adjusted if needed.Telescope fine alignment will beachieved on orbit with the help ofvarious pupil-imaging diagnostic modesincluded in the NIRCam instrument.

Fine pointing of the telescope will beachieved by deflecting the beam bymeans of a fast steering mirrorcontrolled by the Fine Guidance Sensor(provided by the Canadian SpaceAgency) located in the telescope focalplane.

JWST will be operated in a mannersimilar to the HST. The Space TelescopeScience Institute (STScI), whichoperates HST, is under contract toNASA to serve as the operations centrefor JWST and will take on responsibilityfor the scientific exploitation of theobservatory and its instruments aftersuccessful commissioning.

At the time of writing, the JWSTproject is approaching completion ofthe final critical design phase andremains on schedule for a launch in2013.

The NIRSpec InstrumentThe ESA NIRSpec instrument on JWSTis in many ways complementary to theNASA-funded JWST NIRCam near-infrared camera. While NIRCam takesdirect pictures of a patch of sky throughthe JWST telescope, NIRSpec isdesigned to measure the spectra ofpre-selected objects contained in it. Thisreflects the nature of celestial explo-ration: new interesting astronomicalobjects are often first discovered throughimaging, but uncovering their astro-physical properties invariably requiresdetailed follow-up spectro-scopy.

NIRSpec is a multi-object spectro-graph, meaning that it is capable ofmeasuring the spectra of up to 100objects simultaneously. NIRSpec willbe the first such astronomical spectro-graph to fly in space. NIRSpec achievesthis feat thanks to its novel micro-mechanical slit selection device. In thefirst stage of the NIRSpec opticalchain, the field of view to be studied isimaged onto a Micro-Shutter Array(MSA) consisting of just under aquarter of a million individuallyaddressable micro-shutters. The lightfrom the objects under investigation isthen isolated and allowed to enter theinstrument by programming the MSAto only open those shutters coincidingwith the pre-selected objects of interest.The remainder of the optical chainthen serves to separate the light passingthrough the shutters into its componentcolours by means of a prism or adiffraction grating. The resulting

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Chasing the ‘Redshift’

The story of modern astrophysics is one of

ever-growing telescopes and ever-more

sensitive instruments to look deeper into

space. This obsession with the most distant

objects has to do with the fact that the

further away an object, the ‘older’ the light

received, because of the finite speed of

light. Astronomers can therefore ‘look back

in time’ just by looking far enough away.

They can observe directly the conditions in

the Universe billions of years ago. With a

big enough telescope, it is theoretically

possible to map how stars and galaxies

came into being and subsequently evolved

nearly all the way back to the ‘Big Bang’,

some 13.6 billion years ago.

An important consideration for exploration

of the early Universe, however, is the

accompanying ‘redshift’ effect. Because of

the expansion of the Universe, the

wavelength of light emitted by a remote

galaxy becomes stretched during its long travel to Earth. The amount by which the

received wavelength is stretched is determined by how much the Universe expanded in

the time since the light was emitted.

The more distant the galaxy, the greater the redshift. The most remote galaxies known

today have their light redshifted by a factor of nearly 8, meaning that we are viewing

these objects when the Universe was only one eighth its present size. This is equivalent

to looking back some 12.9 billion years into the past, or 95% of the way back to the Big

Bang.

This ‘redshifting’ of light occurs at all wavelengths. To explore the ‘normal’ visible light

emitted by the stars contained in more remote (i.e. younger) galaxies, astronomers have

no choice but to chase this light deep into the infrared. This explains why JWST needs to

both have a larger collecting mirror and be optimised for longer wavelengths compared

to HST.

Redshift is also important because it allows astronomers to sort, by distance and age,

the thousands of remote galaxies detected in very long camera exposures, such as

those made in the Hubble Ultra Deep Field survey. Separating the light of a remote

galaxy into its component colours, by passing it through a prism or reflecting it off a

diffraction grating, allows the intensity of the light received to be measured at each

wavelength. The detailed shape of the resulting ‘spectrum’ of the galaxy enables

astronomers to infer not only the types, but also the ages and chemical composition of

the stars that make up the galaxy. Equally important, the distance to the galaxy can be

determined by measuring the redshift of the spectrum; that is, the amount by which the

observed spectrum is shifted toward the red with respect to how it would look if the same

galaxy were at rest. The task of measuring the redshifts of many faint galaxies

simultaneously is one of the primary design drivers behind the ESA-supplied NIRSpec

instrument on JWST.

The Hubble Ultra-Deep Field image is the farthest look intothe Universe by astronomers to date. Some 10 000 faintgalaxies are visible in this million- second exposure, themost remote of which emitted their light when the Universewas only 5% of its present age. Exploring these galaxiesspectroscopically and probing even further back in time is akey scientific goal of the JWST mission (NASA/ESA/STScI)

spectra are then finally refocused ontoa large format (2k x 4k pixel) low-noiseinfrared detector array where they areregistered and sent to the ground.

NIRSpec carries a total of sixdiffraction gratings and a prism as itsdispersive elements. Depending on thesource brightness and the astrophysicalproblem at hand, these allow the user todisperse the target light by differentamounts, and separate adjacent wave-lengths to a relative accuracy (orspectral resolution) of λ/Δλ = R = 100,1000 or 2750.

In addition to the MSA, a 3 x 3 arcsecIntegral Field Unit and five fixed longslits are also available for detailedspectroscopic studies of isolated singleobjects and other specialised appli-cations.

Another noteworthy feature ofNIRSpec is the use of silicon carbide(SiC) ceramic as basic material for boththe mirrors and the structural parts ofthe instrument. SiC is a unique materialwith a very high stiffness-to-mass ratioand a very high thermal stabilityexpressed through its thermal conduc-tivity to thermal expansion ratio. This

makes the material very suitable for lowtemperature optical applications. SiCwas also used to build the ESAHerschel telescope and is the basicmaterial for the complete telescopestructure and mirrors of the ESA Gaiamission.

NIRSpec is being built by Europeanindustry to ESA’s specifications andmanaged by the ESA JWST Project at


JWST

Europe’s Contributions to the JWST Mission

ESA’s participation in the JWST mission was approved by the ESA Science Programme

Committee in 2003. The four major European contributions to the mission are formalised

in the Memorandum of Understanding on JWST signed by NASA and ESA in 2007:

– provision of the NIRSpec instrument;

– provision of the Optical Bench Assembly of the MIRI instrument through special

funding from the ESA member states;

– provision of the Ariane-5 ECA launcher;

– manpower support to JWST operations.

In return for these contributions, ESA gains full partnership in JWST and secures full

access to the JWST observatory for astronomers from Member States on identical terms

to those of today on the Hubble Space Telescope. European scientists will be

represented on all advisory bodies of the project and will be expected to win observing

time on JWST through a joint peer review process, backed by an expectation of a

minimum ESA share of 15% of the total observing time.

The qualification model of the NIRSpec front optical system undergoing testing (Sagem)

ESTEC. The prime contractor forNIRSpec is EADS Astrium inOttobrunn, Germany. The NIRSpecdetector and MSA subsystems areprovided by NASA’s Goddard SpaceFlight Center.

The MIRI InstrumentIn contrast to the NIRCam, NIRSpecand the Canadian provided TunableFilter instruments that operateexclusively in the near-infrared 1–5 μmwavelength range, the MIRI instrumenton JWST is designed to sample thelonger mid-infrared wavelengths at 5–28 μm. The mid-infrared spectralregion is important astrophysically for anumber of reasons, not least of which isthe ability of mid-infrared light topenetrate the dense interstellar dustclouds that enshroud star formingregions and make them impenetrable tostudy at shorter wavelengths.

As MIRI is the only instrument onJWST sampling mid-infrared wave-lengths, its design is self-contained inthat it carries both a camera mode fordirect imaging and two spectrographmodes providing spectral resolutions ofR=100 and R=2000 respectively. TheMIRI mirrors and structure are made ofaluminium throughout. The instrumentmass is 100 kg.

One noteworthy feature of the MIRIcamera is its novel coronographic mode.

corner points to transfer the launchloads, which also makes the geometrymore compact.

The sunshade that separates the warmspacecraft from the cold telescope isimpressive, reaching the size of a tenniscourt when fully deployed. Thesunshade is stowed in two segmentsalong the front and back of thetelescope during launch. The solararrays and communication antenna dishare also collapsed against the spacecraftbody for launch. After separation fromthe launcher, the observatory willliterally unfold itself like a butterflyfrom its cocoon (see a JWSTdeployment animation available athttp://sci.esa.int/jump.cfm?oid=41816).

The JWST system design is stronglydriven by thermal considerations.Sunlight must be kept off the telescopeand instruments at all cost, and this isachieved with a sunshade made of fiveseparate foils acting as a multi-layerinsulation. The spacecraft, with itsattitude control electronics, powerregulation, communications systems andon-board data handling and processingsystem, is situated on the ‘hot’ side of thesunshade and operate at near-roomtemperature as on most other spacecraft.

The telescope and the instruments onthe ‘cold’ side of the sunshade need tobe kept below –240ºC. This is achievedby physically separating the warmspacecraft from the cold observatory bymeans of a deployable tower that isactivated after launch, thus giving a lowparasitic heat transfer from the warm tothe cold region. All electronics andinstrument parts located in the coldregion must have extremely low powerdissipation in order to maintain the lowtemperatures.

The full complement of four instru-ments with a total mass of 570 kg has anaverage power dissipation of less than0.5 W, less than a small bicycle lamp.The telescope and its instruments arepassively cooled to below –240ºC byexposing them to the deep-spaceenvironment at a temperature of–270ºC. It takes four months to cooldown the observatory after deploymentof the sunshade. This matches very wellthe time it takes to reach the final orbitat L2, where observations can begin.

The Management Challenges of JWSTIn the ESA/NASA cooperation onJWST, mutual responsibilities andobligations are defined at global level in

In this mode, placing the image of a staron a ‘Quadrant Phase Mask’ causes thelight from the star to be dramaticallyattenuated, thereby allowing any planetsorbiting the star to be searched for andimaged directly.

To limit the self-emission from theMIRI instrument and operate its three 1k x 1k SiAs detector arrays, parts of theMIRI instrument need to be cooled some25˚ below that of the passively coolednear-infrared instruments to atemperature less than 10˚ above absolutezero. This is achieved by means of adedicated mechanical cooler.

MIRI is being procured jointly byEurope and the USA. The MIRIOptical System is being built and fundedby a consortium of ESA member statesled by the United Kingdom andcomprised of France, the Netherlands,Germany, Spain, Sweden, Switzerland,Denmark, Belgium and Ireland. Overallleadership of the European MIRIconsortium rests with the EuropeanPrincipal Investigator, Dr GillianWright of the Astronomy TechnologyCentre in Edinburgh. The MIRIdetector arrays and mechanical cryo-cooler are provided by NASA’s JetPropulsion Laboratory.

The Technical Challenges of JWSTThe original name of JWST – the NextGeneration Space Telescope – can beunderstood literally. JWST is the firstspace telescope whose primary mirror islarger than the diameter of the launcherfairing, and the first mission to uselightweight construction techniques andactive adjustment of its mirrors in space.

The large size of the satellite and theneed to fit inside an Ariane-5 launcherfairing requires a number of deployablefeatures. The main telescope and its 18hexagonal mirror segments have twodeployable wings hinged and rotatedagainst the back of the centraltelescope. The secondary mirror issuspended on a tripod in front of themain mirror that is folded and rotatedagainst the back of the primary mirrorduring launch. During launch, thetelescope rests on the spacecraft at four

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Verification Model of MIRI undergoing testing (MIRI EC)


JWST

the JWST Memorandum of Under-standing. The Joint Project Imple-mentation Plan (JPIP) defines themutual responsibilities and obligationin further detail, and also describes themanagement interaction and coordi-nation between the two projects. A keyassumption made in the JPIP is theprinciple of equivalence between theengineering and product assurancestandards to which the two agencieseach work. This makes it possible forthe ESA and NASA contractors towork to standards they are familiarwith.

The JPIP is based on a partnershipand not a contractual relationship. Nomoney flows between the two agencies.As a partner, ESA supports all theprogramme-level activities such asreviews and approval of all higher-levelproject documents. However, NASA isresponsible for the overall missionsystem, and ESA works to interface andfunctional requirements defined byNASA. ESA is responsible for theNIRSpec and the MIRI OpticalSystems. NASA delivers subsystems toboth instruments according to interfaceand functional requirements defined byESA. This makes the overall technicaland programmatic situation rathercomplex

The guiding principle used in thedefinition of the ESA/NASA mutualresponsibilities has been to identify‘Clean and Clear Interfaces’. Thisapplies to both technical and manage-ment interfaces, and has probably beenthe most important aspect to ensure asmooth and efficient cooperation. Anexample is the delivery of detectors forNIRSpec and MIRI from NASA.Detectors, detector electronics, flightsoftware, electrical and mechanicalground support equipment, calibrationand qualification are the responsibilityof NASA. This makes the detectorsystems fully independent subsystemsand easy to manage from both atechnical and programmatic point ofview.

Working cultures are certainlydifferent between ESA and NASA, and JWST in launch configuration inside the Ariane-5 fairing

it is important that the two projectteams on both sides of the Atlantic beconscious of this to avoid conflicts andirritation. The in-depth definition ofdeliverables: hardware, software, ground

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support equipment and documentationcaptured in the JPIP has significantlymitigated conflict situations andsmoothed the cooperation. It has beenmade clear from the beginning what was

The JWST Technology Development Programme

JWST takes us outside the capabilities of today’s technology and satellite design

approach in several areas. Ten critical leading technologies were identified by NASA in

the concept phase of the project. The leading technologies span a very wide range:

mirror segment adjustability with nanometre accuracy, alignment of all 18 mirror

segments, stability of the mirror segment support structure, large format and ultra-low

noise near-infrared and mid-infrared detectors, mechanical coolers for MIRI and the

Micro-Shutter Array for NIRSpec.

All technologies reached the required TRL-6 in spring 2007. TRL-6 requires a system or

subsystem model or prototype demonstration in a relevant environment. This is an

important achievement that allows the JWST project to progress to the implementation

phase with the required technological developments in hand. A significant investment

had been made early in the project development phase to achieve this.

To support the construction of NIRSpec, ESA also faced the need to develop new

technologies in the field of high-performance mirrors and structures, which are

compatible with use at –240oC. The development focused on qualifying silicon carbide

(SiC) ceramic as the basic material for both the mirrors and the structural parts of

NIRSpec.

expected from each party at the lowestpossible level.

The ESA/NASA cooperation is furthercomplicated due to the InternationalTraffic in Arms Regulations (ITAR),which is a set of government regulationsthat control the US export and import ofdefence-related articles and services.Basically all space activities and productsare considered defence-related. Thismakes it very diffcult for a US companyto share any detailed design information,analyses and test procedures with ESA.The exchange of information and serviceneeds to be defined in a TechnicalAssistance Agreement (TAA), which is tobe approved by the US Department ofState (DoS). Typically, very strongprovisos are applied by DoS, whichprecludes any exchange of detaileddesign information. This constraintmakes it even more important to have‘Clean and Clear’ interfaces, with aminimum of exchange of informationrequired. e

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JWST - European Space Agency